check for fill and missing values equality
check for fill and missing values equality

This is related to [compliance-checker issue# (959)](https://github.com/ioos/compliance-checker/issues/959)

If both missing_value and _FillValue be used, they should have the same value. This according to [CF §2.5.1 Recommendations](https://cfconventions.org/Data/cf-documents/requirements-recommendations/conformance-1.9.html).
ioos/compliance-checker,aodn/compliance-checker